The present invention relates to hydrostatic transmission apparatus for a vehicle having at least two drive members disposed one after the other in the direction of travel of said vehicle, the apparatus comprising a main hydraulic pump having two orifices, two main ducts that are respectively a feed main duct and a discharge main duct, first and second hydraulic motors connected to the main pump for the purpose of driving respective ones of said first and second drive members, and a link selector, at least the first hydraulic motor being a dual motor made up of two elementary motors, a series link existing between the first elementary motor of said first hydraulic motor and the second hydraulic motor, the link selector being suitable for taking up a first position in which a series loop including said series link coexists with a direct loop including a direct link linking at least the second elementary motor of the first hydraulic motor to the two respective orifices of the main pump, and a second position in which at least one of said direct and series loops is bypassed by a bypass link.
The vehicle can have two, three, four, or more wheels.
Apparatus of this type is known, for example, from EP 0 547 947, FR 2 719 001, EP 0 816 153, EP 1 010 566, EP 1 026 024, and EP 1 026 025.
The first position of the link selector is, in particular, useful when the vehicle is in a working situation. In such a situation, the series loop makes it possible to synchronize the relevant drive members, while the direct loop makes it possible to obtain the desired output torque. Thus, drive member spin is avoided, while also obtaining the desired output torque, thereby making it possible, in particular for the vehicle to travel over difficult terrain.
When the link selector is in the second position, the total cylinder capacity of the transmission is small, which makes it possible for the vehicle to travel at higher speed. Said second position is thus useful, in particular when the vehicle is traveling on a road for going quickly between two work zones.
Braking of such a vehicle can, at least in part, take place hydrostatically, by pressure being built up in that one of the main ducts that is serving as the discharge at the time.
Naturally, the effect of hydrostatic braking is limited to the group of cylinder capacities of the motors that have a pressure difference at their terminals, with increased pressure at the terminal that is connected to the duct serving as the discharge, and a different pressure at the terminal connected to the duct serving as the fluid feed.
However, when the link selector is in the second position, the bypass link puts certain terminals of motors or of groups of motors at the same pressure, so as to deactivate them. Under such conditions, the hydrostatic braking does not affect the deactivated motor(s). As a result, the retaining torque useful for the braking is merely the retaining torque that is developed by the non-deactivated motor(s). Therefore, the effectiveness of the hydrostatic braking is limited.
An object of the invention is to remedy the above-mentioned drawbacks by proposing hydrostatic transmission apparatus making improved hydrostatic braking possible even when one of the loops constituted by the direct loop and by the series loop is bypassed.
This object is achieved by the fact that the apparatus of the invention further comprises constriction means suitable for being activated to restrict the flow of fluid through said bypass link.
Thus, during hydrostatic braking that is performed while the link selector is in its second position, even the group(s) of cylinder capacities of the motor(s) that is/are deactivated participate(s) in the hydrostatic braking because the constriction in the bypass link puts the terminals of said groups of cylinder capacities at different pressures. In other words, the retaining torque is not only the retaining torque that is developed by the active groups of cylinder capacities, but rather it is also the retaining torque that is obtained by the constriction of the bypass link.
The effectiveness of the hydrostatic braking is thus increased due to the restriction in the flow of fluid through the bypass link.
Advantageously, the constriction means comprise a bypass selector suitable for enabling fluid to flow substantially freely in said bypass link when the fluid pressure in said link is less than a pressure threshold in said link, and for restricting the flow of fluid in said link when said pressure is greater than said pressure threshold.
When in its first position, the bypass selector can thus make it possible for the transmission to operate normally at high speed, while at least one of the direct or the series loops is bypassed, while said bypass selector can be moved to constrict the bypass link when hydrostatic braking is necessary at a level such that the fluid pressure in the bypass link exceeds the pressure threshold.
It should be noted that the bypass link is preferably connected to one of the main ducts and, more precisely, to that one of the main ducts that, when the vehicle is in the preferred direction of travel, is the discharge main duct. In which case, when hydrostatic braking is caused, the pressure in said main duct increases, and causes the pressure in the bypass link to increase, thereby causing the bypass selector to move to restrict the flow through the bypass link when the pressure in said bypass link is greater than the pressure threshold. By choosing the discharge duct to be said duct to which the bypass link is connected, it is possible, when the link selector is in the second position, to prevent the bypassed motor(s) from causing any significant resistive torque to be opposed to the drive torque.
The pressure threshold to which reference is made above can be defined as a function of the additional hydrostatic braking torque necessary on the transmission of the vehicle. Advantageously, said threshold is at least equal to the boost pressure of the hydraulic circuit. For example, it is approximately in the range 10% to 20% of the maximum pressure of the circuit (which is, in general, defined by the pressure ratings of pressure limiters connected to the main ducts).
Advantageously, the apparatus further comprises a check valve that is disposed in parallel with the bypass selector so as to allow free flow in the bypass link, in a single flow direction only.
This possibility is particularly useful when it is desired, in certain operating situations, to sustain a high fluid flow rate in the bypass link, without causing the selector to go into its position in which it restricts the flow through said link.
In particular, as indicated above, the bypass link is preferably connected to that one of the main ducts that serves as the fluid discharge when the vehicle is in the preferred travel direction (forward travel). In certain situations, it can be necessary to reverse at relatively high speed, while the link selector is in its second position and while having relatively large fluid flow-rate requirements. The above-mentioned check valve is then advantageous because, in the direction of travel corresponding to reverse, in which direction the main duct is to which the bypass link is connected serves as the feed, said check valve makes it possible to allow the required flow-rate to pass through, at the feed pressure, without constricting the bypass link.
Advantageously, the bypass selector is a pressure-reducing valve.
Such a valve constitutes simple means for constricting the bypass link in a manner such as to facilitate hydrostatic braking. A pressure-reducing valve placed at one of the terminals of the groups of cylinder capacities inactivated by the link sector being in the second position makes it possible to reduce the pressure at said terminal, thereby generating the pressure difference necessary for said group of cylinder capacities to participate in the hydrostatic braking force.
Advantageously, the apparatus further comprises control means for controlling the bypass selector, which control means are suitable for controlling said selector as a function of the stroke of a control member.
This makes it possible to adjust, in the desired manner, the level of restriction of the flow of fluid through the bypass link.
Advantageously, the constriction of the bypass link is progressive, depending on the position of the bypass selector.
This progressiveness makes it possible, when the link selector is in the second position, to adapt the participation of the deactivated motors in the hydrostatic braking, as a function of the braking needs.
Advantageously, each of the two elementary motors of the first hydraulic motor has first and second elementary connections, while the second hydraulic motor has at least first and second main connections. When the link selector is in the first position, said first elementary connections are then connected to the first main duct, the second elementary connection of the first elementary motor is connected to the second main connection of the second hydraulic motor, while the second elementary connection of the second elementary motor and the first main connection of the second hydraulic motor are connected to the second main duct. When the link selector is in the second position, said first elementary connections are connected to the first main duct and the second elementary connection of the first elementary motor is connected to the second main connection of the second hydraulic motor, while the second elementary connection of the second elementary motor or the first main connection of the second hydraulic motor is connected to the first main duct, via the bypass link.
This makes it possible, in simple manner, to implement the above-mentioned direct loop and series loop.
Advantageously, the first elementary connections are united to form a common first main connection, while the second elementary connections are separate and form respective ones of the second and third main connections.
In this configuration, the motor having said three main connections is particularly compact, and the fact that the two elementary connections are grouped together makes it possible to simplify the circuit.
Under certain circumstances, it is possible to choose that only one of the motors is made up of two elementary motors. Under other circumstances, it is desirable for each of the two motors that serve to drive respective ones of the two drive members situated one after the other to be made up of two elementary motors.